sewer smell from tub

When we flushed the ensuite toilet we can hear it in the bathtub drain. There is also a sewer smell coming from the bathtub drain. This is a new house,1 year old,no septic tank,2 storey. Crawlspace dry,no leaks,no smell. Help!

Sounds like you have a clog somewhere. If so, then this is causing the water from the tub trap to be sucked out, thus leaving a path for sewer gas to escape (no water left in the trap). When you flush, the water creates a vacuum - normally the air is replenished through the vent. If there is a clog, then it still has to get air somewhere - the closest place is the tub. It might be happening in the vanity sink, too, but that trap is higher, and probably has a different access to the vent system.

The bathtub trap, and possibly the shower if you have a separate one that is not used, is dry. That is why you hear the toilet flushing, and also why the sewer odor comes into the room. Run water in the tub for a minute or so.